1. Field of the Invention
This disclosure relates generally to processes for the production of elemental sulfur from sulfur dioxide, and more particularly to processes for the recovery of sulfur from effluent streams containing sulfur dioxide.
2. Description of the Related Art
Sulfur dioxide is found in many industrial gases emanating from plants involved in roasting, smelting and sintering sulfide ores, or gases from power plants burning high sulfur coal or fuel oils or other sulfurous ores or other industrial operations involved in the combustion of sulfur-bearing fuels, such as fuel oil. One of the more difficult environmental problems facing industry is how to economically control SO2 emissions from these sources.
Several processes schemes have been developed to recover elemental sulfur from SO2 streams. There are several fundamental problems common to these efforts. In particular, byproduct formation of H2S, CS2, COS, H2 and CO reduces sulfur recovery and fuel efficiency and requires larger equipment because of the increased gas flow. Soot formation reduces the quality of the sulfur product and fouls the equipment and catalyst beds reducing the reliability of the unit.
Thermal Reduction of SO2 to Sulfur was developed during 1909-11, S. W. Young investigated reduction of SO2 with methane and other hydrocarbons on a laboratory scale, (Fleming, E. P., and Fitt, T. C., High Purity Sulfur from Smelter Gases—Reduction with Natural Gas, Ind. Eng. Chem., 42, 2249-2253, November 1950). In a 1934 article, Yushkevich, and others, discuss in detail the various possible reaction products from the combination of SO2 and a hydrocarbon reducing agent, including H2S, COS, CS2 and sulfur. Experiments suggested 900-1000° C. as the optimum temperature. (Yushkevich et al, Article Z H. KHIM. PROM., No. 2, 33-37, 1934. and U.S. Pat. No. 3,928,547, Process for the Reduction of Sulfur Dioxide, Daley, W. D., Wilkalis, J. E., and Pieters, W. J. M., Allied Chemical Corp., Dec. 23, 1975). In 1938, American Smelting and Refining Company (ASARCO) initiated investigations, which soon indicated that relatively low-grade SO2 might be directly converted to reasonably pure sulfur by reduction with natural gas. (Fleming, E. P., and Fitt, T. C., High Purity Sulfur from Smelter Gases—Reduction with Natural Gas, Ind. Eng. Chem., 42, 2249-2253, November 1950). Laboratory and small-scale pilot operations were gradually expanded until a semi-commercial 5-tpd unit was operated during 1940-45. Gas from copper roasters or converters containing 5-8% SO2 and 9-12% oxygen was combusted with sufficient natural gas to consume all the oxygen to CO2, plus additional fuel to react with an appropriate portion of the SO2 according to the following overall reaction with CH4 as shown in the following reaction:2SO2+CH4→2H2O+CO2+S2 
Considerable quantities of byproduct H2S, COS and CS2 were formed as well. Furnace temperatures of at least 1250° C. were considered necessary to minimize soot, which will discolor the sulfur. The gases were then cooled and passed through a series of Claus stages for hydrolysis of COS and CS2 to H2S and reaction of residual H2S and SO2 to sulfur according to the Claus reaction. This process is still employed today where potential sulfuric acid supply exceeds demand. In 1978, Davy Power gas GmbH proposed a staged combustion process where hydrocarbon gas is burned at near stoichiometric conditions, followed by injection of supplemental CH4 and SO2 which react to form elemental sulfur. (U.S. Pat. No. 4,117,100, Hellmer, L., Koller, G., Muddarris, G. R. A., and Sud, K. K., Process for Reduction of Sulfur Dioxide to Sulfur, Davy Powergas GmbH, Sep. 26, 1978). It is also claimed that the presence of water vapor in the SO2 feed stream suppresses soot formation. The process was never commercialized.
Catalytic Reduction of SO2 to Sulfur was considered in a 1934, United Verde Copper Company proposed a process where a portion of the SO2 stream is combined with CH4 at 800-850° C. in the presence of a metal sulphide catalyst to produce H2S, which is subsequently reacted with the remaining SO2 to yield sulfur according to the Claus reaction. (U.S. Pat. No. 1,967,263, Rosenstein, L., Recovery of Sulfur, United Verde Copper Company, Jul. 24, 1934). The Claus stage preferably comprised a bed of granular absorbent, such as bauxite or charcoal, continually wetted by a thin film of liquid water which served to absorb the reaction heat and also carry away the product sulfur for subsequent recovery by filtration or sedimentation. The process was never commercialized. In 1965, Texas Gulf Sulfur patented the reduction of SO2 with hydrocarbons (e.g.: CH4) at 750-1000° C. using a catalyst such as alumina, initially achieving 40-60% sulfur recovery (U.S. Pat. No. 3,199,955, West, J. R., and Conroy, E. H., Process of Reducing Sulfur Dioxide to Elemental Sulfur, Aug. 10, 1965). Two similar catalytic stages typically followed, whereby the second stage achieved at 390° C., with the sequence of Hydrolysis of byproduct COS and CS2 to H2S, Claus reaction of H2S and SO2 to form sulfur and Reduction of SO2 by CO and H2 to sulfur. Claus reaction of residual H2S and SO2 further proceeded in the third stage for 95% overall sulfur recovery. No method of controlling the heat release from the reduction reactions is described and the process was never commercialized.
In 1975, Allied Chemical Corp. claimed to have discovered that, at SO2 concentrations on the order of 50% and higher, a small amount of elemental sulfur (0.1-3 mol-% of the feed gas as S8) lowered the initiation temperature for SO2 reduction and favorably moderated the temperature rise and rate. The sulfur also expedited the reaction and minimized byproduct H2, CO, COS and CS2 formation, (U.S. Pat. No. 3,928,547, Process for the Reduction of Sulfur Dioxide, Daley, W. D., Wilkalis, J. E., and Pieters, W. J. M., Allied Chemical Corp., Dec. 23, 1975). Generation of H2 and CO is particularly counterproductive because it decreases sulfur recovery and fuel efficiency and requires larger equipment because of the increased tail gas volume. In 1977, Allied Chemical presented an innovative 3-bed arrangement that was claimed to optimize reactant concentrations and temperatures, (U.S. Pat. No. 4,039,650, Daley, W. D., Sulfur Dioxide Reduction, Allied Chemical Corp., Aug. 2, 1977). The total SO2 stream is mixed with a portion of the CH4 and passed through the first reactor to effect reduction of a portion of the SO2 to H2S and sulfur. Exit gas from the first reactor is mixed with the remaining CH4, and the resultant mixture split into two gas streams which are passed, in parallel, through a second and third reactor to further effect reduction of SO2 to H2S and sulfur. Periodically, the flow in the first and third reactors is reversed to subject them to alternating heat absorbing and desorbing cycles (while the second reactor is always maintained in the same direction). Inlet gas temperatures to the second and third reactors are maintained within desired ranges by bypassing a portion of the SO2 and CH4 around the first reactor. A 25-tpd pilot plant was constructed in 1978 at a 115-MW coal-fired power plant.
The Catalytic Reduction of Sulfur to Intermediate H2S was considered. Early research on the recovery of sulfur from gypsum (CaSO4.2H2O) involved reduction roasting of gypsum with coal or natural gas to form calcium sulfide, which was subsequently processed to generate H2S. In the laboratory, elemental sulfur was then produced by reacting H2S with SO2 at ambient temperature in a liquid medium. That latter concept led the Federal Bureau of Mines, beginning in 1968, to consider absorption of SO2 (from nonferrous smelters) in a liquid medium subsequently regenerated with H2S to precipitate sulfur. After screening many reagents, an aqueous solution of citric acid neutralized with soda ash to a pH of 4.5 was selected, (Crocker, L., Martin, D. A., and Nissen, W. I., Citrate-Process Pilot-Plant Operation at the Bunker Hill Company, Bureau of Mines Report of Investigations 8374, p. 1-6, 1979). At least three pilot plants were operated during 1971-76. The most recent was located at the Bunker Hill Co.'s lead smelter in Kellogg, Id. In the absence of an external source, H2S was generated by the reaction of natural gas with sulfur vapor at 650° C. over a proprietary catalyst as shown in the following reaction:CH4+4S→CS2+2H2S
The product CS2 was subsequently hydrolyzed with steam in a second catalytic stage at 315° C. as shown in the following reaction:2H2S+CS2+2H2O→4H2S+CO2 
The so-called Citrate Process for Claus reaction of H2S and SO2 within a liquid absorbent was ultimately abandoned due to absorber corrosion and plugging problems. (Kohl, A. L., and Nielsen, R. B., Gas Purification, Fifth Edition, p. 564, Gulf Publishing Co., 1997).
During 1978-1980, a series of three U.S. patents by Beavon, D. K., as described below, proposed innovations to reduce equipment costs and improve operability and product quality. A common theme is the efficient reduction of recycled sulfur to H2S for subsequent reaction with SO2 to produce sulfur, while minimizing the soot formation characteristic of direct SO2 reduction. Sulfur Reduction by Submerged Hydrocarbon Combustion was in a 1978 patent, H2 and CO are initially formed in a reducing gas generator by the partial combustion of a hydrocarbon fuel, with steam injection to suppress soot formation. The fuel can be gaseous (such as methane), liquid (such as kerosene, diesel or other fuel oil) or solid (such as coal or coke), (U.S. Pat. No. 4,094,961, Beavon, D. K., Hydrogen Sulfide Production, Ralph M. Parsons Company, Jun. 13, 1978).
The reducing gas is sparged through molten sulfur, so that combustion temperatures are rapidly quenched by sulfur vaporization. The firing rate is adjusted to produce a 250-450° C. vapor stream with a nominal stoichiometric excess of hydrogen, which is then passed across a fixed cobalt-moly catalyst bed. Elemental sulfur is hydrogenated to H2S. Byproduct COS and CS2 are hydrolyzed to H2S, and CO is hydrolyzed to CO2 and H2. Sufficient reaction heat is generated that multiple beds with inter-stage cooling are typically required. Reactor effluent is cooled in the sulfur cooler to condense any residual sulfur vapor, particularly during non-routine operation, while remaining above the water dew point. The gas is then further cooled to condense most of the water vapor, yielding an H2S-rich stream that can then be reacted with SO2 in a conventional Claus reactor to produce elemental sulfur. The process has not been commercialized. Sulfur Reduction in a Reaction Furnace was in a 1979 patent, hydrogen and CO are similarly generated by partial oxidation of a hydrocarbon, gaseous or liquid, in the first zone of a 2-zone furnace, and a stoichiometric excess of liquid sulfur is injected into the second zone to quench temperatures to 800-1100° C., (U.S. Pat. No. 4,146,580, Beavon, D. K., Process for Hydrogen Sulfide Production, Ralph M. Parsons Company, Mar. 27, 1979).
A portion of the H2 and CO react with the sulfur to form H2S, COS and some CS2, with about 50% of the total H2S production being achieved in the furnace. The resultant vapor stream is rapidly cooled to 425° C. or less in a waste heat boiler to suppress further formation of undesirable organic sulfur byproducts. The stream is then further cooled to condense and remove most of the residual sulfur. The gas stream is then typically be reheated for conventional catalytic hydrogenation of sulfur and SO2 to H2S, hydrolysis of COS and CS2 to H2S and hydrolysis of CO to CO2 and hydrogen. The reactor effluent is then cooled by conventional means to ultimately condense most of the water vapor, yielding an H2S-rich gas stream that can be subsequently reacted with SO2 in a conventional Claus reactor to yield elemental sulfur. As with the previous process, this process has not been commercialized.
Thermal Reduction of SO2 was developed in a 1980 patent, a hydrocarbon fuel, gaseous or liquid, is partially oxidized in a reaction furnace to generate H2 and CO and SO2 added to the thermal reaction zone to react with the H2 and indirectly, CO (by virtue of water gas shift to CO2 and H2). The firing rate is adjusted to yield a mixture of H2S and SO2 in the molar ratio of 2:1 as required by Claus stoichiometry (U.S. Pat. No. 4,207,304, Beavon, D. K., Process for Sulfur Production, Ralph M. Parsons Company, Jun. 10, 1980).
Competing reactions are the formation of COS and CS2 from the reaction of CO and free carbon with SO2 and sulfur. Potential soot may be washed from the system by the introduction of liquid sulfur, which is recycled to enable consumption of extracted carbon. The resultant vapor stream is rapidly cooled to 425° C. or less to suppress further formation of undesirable organic sulfur byproducts. Elemental sulfur is recovered and recycled to the reactor for gasification of extracted carbon solids and tars.
Further sulfur recovery is achieved as the process gas proceeds through a series of conventional catalytic Claus stages.
In 1965, the Ontario Research Foundation began investigating removal of SO2 from smelter off-gases. A process was proposed wherein sulfur dioxide is removed from a sulfur dioxide containing effluent gas by reacting the sulfur dioxide in the effluent gas with a reducing gas comprising hydrogen sulfide in a presence of a catalyst in accordance with the equation:SO2+2H2S→3S2+2H2Oat a temperature of at least 50° C. The reducing gas is formed by reaction of hydrocarbon fuel, steam and sulfur at a temperature of about 500° C. to about 900° C. The temperature is generated by the combustion of hydrocarbon fuel (Canadian patent 846,697, Removal of Sulphur Dioxide from Waste Gases, Sefton, V. B., Wilkalis, Ontario Research Foundation, Jul. 14, 1970).
A disadvantage of the Ontario Research Foundation process is that a large amount of steam is required to minimize byproduct COS or CS2 in the reducing gas. Accordingly, a large amount of water and energy are consumed in order to provide steam to the process. The water can also have the effect of limiting the conversion of SO2 to sulfur in the above reaction.
In a similar process, a reducing agent and preheated sulfur at a temperature of 500° C. to 650° C. are contacted in a furnace to produce a reducing gas that comprises H2S and CS2. The reducing gas is mixed with an SO2 containing stream and contacted by a catalyst in a reactor wherein sulfur is produced and CS2 is concurrently hydrolyzed to SO2 and CO2. (U.S. Pat. No. 7,485,281 B2, Rameshni, M., Process for the Production of Sulfur from Sulfur Dioxide, Feb. 3, 2009).
One disadvantage of this process is that the sulfur must be preheated to 500° C. to 650° C. before mixing with the reducing agent to improve reaction kinetics. Such high temperature may cause increased corrosion of the process equipment.
Another disadvantage is that a high concentration of CS2 is produced in the H2S generator. CS2 is known to be a highly toxic chemical and an explosion hazard with an exceptionally low auto ignition temperature of 90° C. in air, significantly below the operating temperature of the process.
This application for patent discloses processes for the production of elemental sulfur from sulfur dioxide.